1. Field of the Invention
This invention relates to the separation of gases and is particularly concerned with the removal of gaseous sulfur and nitrogen compounds from other gases by contacting the gases with liquid organic disulfides.
2. Description of the Prior Art
There are many situations which require the separation of hydrogen sulfide, sulfur oxides, nitrogen oxides, or similar compounds from other gases. In some cases one or more such compounds may be present in relatively high concentrations and recovered as the principal product but in most instances these compounds are present in relative low concentrations and are regarded as contaminants which must be removed before the other gases can be further processed or vented to the atmosphere. This is particularly true today because the majority of the sulfur and nitrogen-containing compounds are considered harmful air pollutants which cannot be discharged into the atmosphere. Contaminants of particular concern are hydrogen sulfide, nitrogen dioxide and sulfur dioxide. It is often desirable to selectively remove these contaminants from gas streams containing carbon dioxide.
There are numerous commercial processes which can be used to remove hydrogen sulfide and other acid gases from natural, refinery, and synthesis gas streams. Several of these processes utilize an aqueous solution of an alkanolamine as a solvent to absorb the gaseous contaminants. A monoethanolamine solution is used to remove both hydrogen sulfide and carbon dioxide. If it is also desirable to remove carbonyl sulfide and carbon disulfide, a diethanolamine solution can be used. A certain degree of selectivity in the presence of carbon dioxide may be obtained by using a triethanolamine solution but this solvent is more expensive than the others.
Another group of processes utilizes solvents composed of alkaline salt solutions, usually a potassium carbonate solution, in combination with various additives and catalysts. These processes are normally used to remove hydrogen sulfide, carbon dioxide, carbonyl sulfide and carbon disulfide from contaminated gas streams. Only a few of these processes can be employed to selectively remove hydrogen sulfide in the presence of carbon dioxide.
There are other commercial processes available which utilize sophisticated organic solvents as absorbents. One such process employs the dimethyl ether of polyethylene glycol, which has a strong preference for most sulfur-containing compounds and the capacity to absorb large quantities of most impurities economically. Still other proposed processes for treating gas streams involve, for example, the removal of both sulfur dioxide and hydrogen sulfide from flue gas streams by contacting the streams with an aqueous solution of sodium carbonate and then reacting the resulting solution with ammonium hydrogen carbonate.
The processes referred to above and similar techniques have pronounced disadvantages. The solvent utilized to absorb the gaseous impurities may be expensive, may have an affinity for only a few contaminants, may exhibit a high absorptive capacity but not be selective in the presence of carbon dioxide or may have a high selectivity for particular compounds but show little absorptive capacity. None of the solvents which have been proposed heretofore have the ability to absorb large quantities of hydrogen sulfide, sulfur oxides and nitrogen oxides while still exhibiting a high selectivity for such compounds in the presence of carbon dioxide. It should also be noted that many of the commercial and proposed processes are undesirable because they are overly complicated, utilize cumbersome equipment, require the maintenance of high pressure and low temperatures, or are expensive to operate.